It is known that there are many applications for attenuator circuits. One example of an application for such a circuit is a modulator wherein an rf input signal is selectively attenuated as a function of a control or modulating dc current to provide amplitude modulation.
It has been known that certain components such as, for example, PIN diodes, can exhibit the properties of a variable resistor at microwave frequencies which are too high for rectification to take place because of the relatively large recovery time of a fixed i (intrinsicly doped) layer. At zero or reverse bias, the i layer introduces a high resistance. Under forward bias, however, the injection and storage of carriers reduces the resistance of the i region according to R.sub.i =(20-50)/i.sup.0.88 ohms, where i is the forward dc bias current in milliamps. Some prior art attenuators have been designed using this knowledge with such components as PIN diodes and FET's to accomplish rf attenuation which is a function of the dc bias current through the component. Such circuits have also frequently utilized a constant dc current source to control the dc bias current. It is also noted that it is common in the prior art to use balanced mixers to provide rf attenuation in the design of modulators and complex modulators. A substantial disadvantage of both the prior PIN diode approach and the use of balanced mixers is that the circuits heretofore used exhibit a limited range of attenuation linearity for various dc bias currents. Typically, the range of linear response has been on the order of 30 db or less. There is still a need for a circuit that provides substantial linear attenuation for a dc bias current range of greater than 50 db.